WMOD00385

• Title: Augmentation Potential through Weather Modification - Working Document
• Date: 2/1/1975
• Weather Modification - Doc Type: Report

Accurate targeting - that is, putting the additional precipitation
in a predetermined area on the ground - requires an understanding of
how the seeding material travels from the generator to the cloud,
how long it takes to develop a precipitation-size ice crystal, and
the trajectory of the ice crystal as it falls out of the cloud. The
stabili ty of the atmosphere, the average windspeed and direction,
and the deviation from these average values are the major factors
in targeting. Sometimes local situations, such as the close prox-
imi ty of population centers or specific areas requested to be
excluded, require confining the seeding effects to a rather pre-
cisely defined target area. In these cases, the windspeed and
direction must be monitored very carefully. Several mathematical
'models are available to help predict the area affected by cloud
seeding.

Orographic clouds typically form hours before the first precipita-
tion is observed and. frequently last many hours after precipita-
tion stops. Cloud seeding can be used to increase the duration as
well as increase the intensity of precipitation. If clouds are
seeded as soon as favorable conditions exist, precipitation may
start earlier, last longer, and be of greater intensity. Many
orographic clouds are reasonably efficient precipitators once they
get started, so if seeding is delayed until natural precipitation
is observed, the intensity may be increased somewhat and the dura-
tion extended, but the total precipitation will not be as great as
if the seeding had preceded natural precipitation.

Evaluation. - The results of seeding orographic clouds can be
expressed in terms of precipitation, runoff, streamflow, or a host
of nonhydrologic and nonmeteorological factors. A number of stand-
ard statistical techniques are available to analyze the data, and.
several special techniques have been studied. Evaluation tech-
niques generally fall into one or the other of two categories -
target vs. control and seed vs. no seed. In the target-control
design, all storms affecting the target area are seeded and a
nearby control area is never seeded. The target and the control
areas must experience the same climate and the averages of pre-
cipitation falling in the two areas must be highly correlated. The
precipitation in the control area is made a predictor of what the
target area would have received if there had been no seeding. The
difference between actual and predicted target precipitation is a
measure of the effect of the seeding. Runoff, streamflow, or other
variables could be used as predictors instead of precipitation. The
seed-no seed design requires that a percentage of all seedable
storms be left unseeded. The selection of the unseeded storms must

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